Verification metrology targets and their design

ABSTRACT

Metrology target design methods and verification targets are provided. Methods comprise using OCD data related to designed metrology target(s) as an estimation of a discrepancy between a target model and a corresponding actual target on a wafer, and adjusting a metrology target design model to compensate for the estimated discrepancy. The dedicated verification targets may comprise overlay target features and be size optimized to be measureable by an OCD sensor, to enable compensation for inaccuracies resulting from production process variation. Methods also comprise modifications to workflows between manufacturers and metrology vendors which provide enable higher fidelity metrology target design models and ultimately higher accuracy of metrology measurements.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of and claimspriority to U.S. patent application Ser. No. 15/351,995, filed on Nov.15, 2016, which claims priority to PCT/US2015/053838, filed on Oct. 2,2015, which claims the benefit of U.S. Provisional Patent ApplicationNo. 62/059,640 filed on Oct. 3, 2014, whereby each of the patentapplications listed above are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION 1. Technical Field

The present invention relates to the field of semiconductor metrology,and more particularly, to metrology target design.

2. Discussion of Related Art

In the field of semiconductor metrology, a metrology tool may comprisean illumination system which illuminates a target, a collection systemwhich captures relevant information provided by the illuminationsystem's interaction (or lack thereof) with a target, device or feature,and a processing system which analyzes the information collected usingone or more algorithms. Metrology tools can be used to measurestructural and material characteristics (e.g., material composition,dimensional characteristics of structures and films such as filmthickness and/or critical dimensions of structures, overlay, etc.)associated with various semiconductor fabrication processes. Thesemeasurements are used to facilitate process controls and/or yieldefficiencies in the manufacture of semiconductor dies. Metrology toolsmay include one or more hardware configurations which may be used inconjunction with certain embodiments of this invention to, e.g., measurethe various aforementioned semiconductor structural and materialcharacteristics. Examples of such hardware configurations include thefollowing: a spectroscopic ellipsometer (SE), a SE with multiple anglesof illumination, a SE measuring Mueller matrix elements (e.g., usingrotating compensator(s)), a single-wavelength ellipsometers, a beamprofile ellipsometer (angle-resolved ellipsometer), a beam profilereflectometer (angle-resolved reflectometer), a broadband reflectivespectrometer (spectroscopic reflectometer), a single-wavelengthreflectometer, an angle-resolved reflectometer, any imaging system, apupil imaging system, a spectral imaging system, a scatterometer (e.g.,speckle analyzer) etc.

The hardware configurations can be separated into discrete operationalsystems. On the other hand, one or more hardware configurations can becombined into a single tool. One example of such a combination ofmultiple hardware configurations into a single tool is provided by U.S.Pat. No. 7,933,026 (including e.g., a broadband SE, a SE with rotatingcompensator, a beam profile ellipsometer, a beam profile reflectometer,a broadband reflective spectrometer, and a deep ultra-violet reflectivespectrometer) which is incorporated herein by reference in its entirety.In addition, there are typically numerous optical elements in suchsystems, including certain lenses, collimators, mirrors, quarter-waveplates, polarizers, detectors, cameras, apertures, and/or light sources.The wavelengths for optical systems can vary from about 120 nm to 3microns. For non-ellipsometer systems, signals collected can bepolarization-resolved or unpolarized. Multiple metrology heads may beintegrated on the same tool, however, in many cases, multiple metrologytools are used for measurements on a single or multiple metrologytargets, as described e.g., in U.S. Pat. No. 7,478,019, which isincorporated herein by reference in its entirety.

The illumination system of the certain hardware configurations includesone or more light sources. The light source may generate light havingonly one wavelength (i.e., monochromatic light), light having a numberof discrete wavelengths (i.e., polychromatic light), light havingmultiple wavelengths (i.e., broadband light) and/or light that sweepsthrough wavelengths, either continuously or hopping between wavelengths(i.e., tunable sources or swept source). Examples of suitable lightsources are: a white light source, an ultraviolet (UV) laser, an arclamp or an electrode-less lamp, a laser sustained plasma (LSP) source, asupercontinuum source (such as a broadband laser source), orshorter-wavelength sources such as x-ray sources, extreme UV sources, orsome combination thereof. The light source may also be configured toprovide light having sufficient brightness, which in some cases may be abrightness greater than about 1 W/(nm cm² Sr). The metrology system mayalso include a fast feedback to the light source for stabilizing itspower and wavelength. Output of the light source can be delivered viafree-space propagation, or in some cases delivered via optical fiber orlight guide of any type.

The metrology targets may possess various spatial characteristics andare typically constructed of one or more cells which may includefeatures in one or more layers which may have been printed in one ormore lithographically distinct exposures. The targets or the cells maypossess various symmetries such as two fold or four fold rotationsymmetry, reflection symmetry, as described e.g., in U.S. Pat. No.6,985,618, which is incorporated herein by reference in its entirety.Different cells or combinations of cells may belong to distinct layersor exposure steps. The individual cells may comprise either isolatednon-periodic features or alternately they may be constructed from one,two or three dimensional periodic structures or combinations ofnon-periodic and periodic structures as e.g., in U.S. Patent PublicationNo. 2013/042089, which is incorporated herein by reference in itsentirety. The periodic structures may be non-segmented or they may beconstructed from finely segmented features which may at or close to theminimum design rule of the lithographic process used to print them. Themetrology targets may also be collocated or in close proximity withdummification structures in the same layer or in a layer above, below orin between the layers of the metrology structures. Targets can includemultiple layers (or films) whose thicknesses can be measured by themetrology tool. Targets can include target designs placed (or alreadyexisting) on the semiconductor wafer for use, e.g., with alignmentand/or overlay registration operations. Certain targets can be locatedat various places on the semiconductor wafer. For example, targets canbe located within the scribe lines (e.g., between dies) and/or locatedin the die itself. Multiple targets may be measured (at the same time orat differing times) by the same or multiple metrology tools as describede.g., in U.S. Pat. No. 7,478,019, which is incorporated herein byreference in its entirety. The data from such measurements may becombined. Data from the metrology tool is used in the semiconductormanufacturing process for example to feed-forward, feed-backward and/orfeed-sideways corrections to the process (e.g., lithography, etch), seee.g., U.S. Pat. No. 8,930,156, which is incorporated herein by referencein its entirety, disclosing feed forward methods for reusing metrologytarget cells; and therefore, might yield a complete process controlsolution. The metrology tools are designed to make many different typesof measurements related to semiconductor manufacturing, for examplemeasure characteristics of one or more targets, such as criticaldimensions, overlay, sidewall angles, film thicknesses, process-relatedparameters (e.g., focus and/or dose). The targets can include certainregions of interest that are periodic in nature, such as for examplegratings in a memory die.

As semiconductor device pattern dimensions continue to shrink, smallermetrology targets are often required. Furthermore, the measurementaccuracy and matching to actual device characteristics increase the needfor device-like targets as well as in-die and even on-devicemeasurements. Various metrology implementations have been proposed toachieve that goal. For example, focused beam ellipsometry based onprimarily reflective optics is described e.g., in U.S. Pat. No.5,608,526, which is incorporated herein by reference in its entirety.Apodizers can be used to mitigate the effects of optical diffractioncausing the spread of the illumination spot beyond the size defined bygeometric optics, as described e.g., in U.S. Pat. No. 5,859,424, whichis incorporated herein by reference in its entirety. The use ofhigh-numerical-aperture tools with simultaneous multipleangle-of-incidence illumination is another way to achieve small-targetcapability, as described e.g., in U.S. Pat. No. 6,429,943 which isincorporated herein by reference in its entirety. Other measurementexamples may include measuring the composition of one or more layers ofthe semiconductor stack, measuring certain defects on (or within) thewafer, and measuring the amount of photolithographic radiation exposedto the wafer. In some cases, metrology tool and algorithm may beconfigured for measuring non-periodic targets, as described e.g., inU.S. patent application Ser. No. 14/294,540 and in U.S. PatentPublication No. 2014/0222380, which are incorporated herein by referencein their entirety.

Measurement of parameters of interest usually involves a number ofalgorithms, carried out by corresponding analysis units in therespective metrology tools. For example, optical interaction of theincident beam with the sample is modeled using EM (electro-magnetic)solver and uses such algorithms as RCWA (Rigorous Coupled WaveAnalysis), FEM (finite element method), method of moments, surfaceintegral method, volume integral method, FDTD (Finite Difference TimeDomain), and others. The target of interest is usually modeled(parametrized) using a geometric engine, or in some cases, processmodeling engine or a combination of both. The use of process modeling isdescribed e.g., in U.S. Patent Publication No. 2014/0172394, which isincorporated herein by reference in its entirety. A geometric engine isimplemented, for example, in AcuShape software product of KLA-Tencor.

Collected data can be analyzed by a number of data fitting andoptimization techniques an technologies including libraries,Fast-reduced-order models; regression; machine-learning algorithms suchas neural networks, support-vector machines (SVM);dimensionality-reduction algorithms such as, e.g., PCA (principalcomponent analysis), ICA (independent component analysis), LLE(local-linear embedding); sparse representation such as Fourier orwavelet transform; Kalman filter; algorithms to promote matching fromsame or different tool types, and others. Collected data can also beanalyzed by algorithms that do not include modeling, optimization and/orfitting modeling as described e.g., in U.S. Patent Publication No.2014/0257734, which is incorporated herein by reference in its entirety.Computational algorithms are usually optimized for metrologyapplications with one or more approaches being used such as design andimplementation of computational hardware, parallelization, distributionof computation, load-balancing, multi-service support, dynamic loadoptimization, etc. Different implementations of algorithms can be donein firmware, software, FPGA (Field Programmable Gate Array),programmable optics components, etc. The data analysis and fitting stepsusually pursue one or more of the following goals: Measurement of CD,SWA, shape, stress, composition, films, bandgap, electrical properties,focus/dose, overlay, generating process parameters (e.g., resist state,partial pressure, temperature, focusing model), and/or any combinationthereof; modeling and/or design of metrology systems; and modeling,design, and/or optimization of metrology targets;

SUMMARY OF THE INVENTION

The following is a simplified summary providing an initial understandingof the invention. The summary does not necessarily identify key elementsnor limits the scope of the invention, but merely serves as anintroduction to the following description.

One aspect of the present invention provides a metrology target designmethod comprising using OCD data related to at least one designedmetrology target as an estimation of a discrepancy between a targetmodel and a corresponding actual target on a wafer, and adjusting ametrology target design model to compensate for the estimateddiscrepancy.

These, additional, and/or other aspects and/or advantages of the presentinvention are set forth in the detailed description which follows;possibly inferable from the detailed description; and/or learnable bypractice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of embodiments of the invention and to showhow the same may be carried into effect, reference will now be made,purely by way of example, to the accompanying drawings in which likenumerals designate corresponding elements or sections throughout.

In the accompanying drawings:

FIGS. 1 and 2 are high level flowcharts illustrating metrologyworkflows, according to some embodiments of the invention.

FIG. 3A is a high level schematic illustration of a prior art overlaymetrology target.

FIGS. 3B and 3C are high level schematic illustrations of verificationtargets, according to some embodiments of the invention.

FIG. 4 is a high level schematic flowchart illustrating a target designmethod, according to some embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With specific reference now to the drawings in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

Before at least one embodiment of the invention is explained in detail,it is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement of thecomponents set forth in the following description or illustrated in thedrawings. The invention is applicable to other embodiments that may bepracticed or carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein is for the purposeof description and should not be regarded as limiting.

Metrology target design methods and verification targets are provided.Methods comprise using OCD data related to designed metrology target(s)as an estimation of a discrepancy between a target model and acorresponding actual target on a wafer, and adjusting a metrology targetdesign model to compensate for the estimated discrepancy. The dedicatedverification targets may comprise overlay target features and be sizeoptimized to be measureable by an OCD sensor, to enable compensation forinaccuracies resulting from production process variation. Methods alsocomprise modifications to workflows between manufacturers and metrologyvendors which provide enable higher fidelity metrology target designmodels and ultimately higher accuracy of metrology measurements.

FIGS. 1 and 2 are high level flowcharts illustrating metrologyworkflows, according to some embodiments of the invention.

Metrology targets need to be designed in accordance with specificmetrology performance requirements such as accuracy, precision androbustness. In order to enable these requirements simulations are oftenused in order to determine the relationship between the geometriccharacteristics of the target design and the resultant metrologyperformance as described e.g., in U.S. Pat. No. 8,214,771 which isincorporated herein by reference in its entirety. Typically, thesimulation procedure begins with the insertion of a nominal stack andtopography description of the semiconductor wafer in general and of themetrology target in particular.

The reliability of the simulation in predicting the metrologyperformance of a given metrology design is strongly influenced by theaccuracy of the nominal stack and topography description. In thefollowing, methods and target designs are presented, which enableimproved accuracy in the prediction of metrology performance byverification of the stack and topography description. Parameters, suchas target design characteristics, which may be verified usingembodiments of the methods, include, but are not limited to the presenceor absence or value of film stack thicknesses, film stack sequence,optical indices, e.g., the refractive indices (n) and extinctionco-efficients (k), period, dispersion, duty cycle, critical dimension,critical dimension etch bias, induced topography, dishing due to over(or under) polish, side wall angles, asymmetry of side wall angles,layer planarity or lack thereof, LER (Line Edge Roughness), or any othertopographic or optical characteristic which may impact metrologyperformance.

Certain embodiments of stack and topography verification sequences areshown in FIG. 1 . Prior art workflow characteristically starts withnominal stack input from customer questionnaire 61, concerning, e.g., n,k and t (topography) parameters of the stack, and received from asemiconductor manufacturer 60. A metrology vendor 70 conducts apreliminary stack review 71, which is then verified 72 by comparisonwith metrology target cross section image(s) 62 provided by manufacturer60. The stack description may be updated 73 upon receipt of processvariation estimates 63, and target design (target design of experimenttDOE) workbook 74 is prepared, target design simulations 75 are run andpupil images, diffraction efficiency (DE), and precision data 76 areacquired. Target coordinates 64 and metrology tooltime 65 are receivedfrom manufacturer 60, metrology verification recipes are prepared 81,measured 82 and used to derive pupil images, DE, and precision data 83.The estimated and measured results 76, 83 respectively are compared 84to evaluate the matching of the targets to the specification. In case ofmismatch, stack description is updated again 73, new workbook(s),simulations and data are prepared, and simulations 85 are carried out,resulting in verified target designs 86.

In certain embodiments, an additional earlier verification step isadded, namely a comparison with OCD (Optical Critical Dimension) andfilm workbooks 100, received e.g., from OCD workbook and results forprevious layer(s) 110, film workbook and results for lithographiclayer(s) 112 and/or film workbook and results for previous layer(s) 114,followed by an updated stack description 116 carried out possibly evenbefore verification step 72 (concerning metrology target cross section62). The OCD data used relates to designed metrology target(s) andprovides an estimation of a discrepancy between device parameters andtarget parameters and/or an estimation of a discrepancy between designedtarget parameters and actual target parameters (of the targets asprinted on the wafer). Updating 116 may adjust the respective metrologytarget design(s) to compensate for the estimated discrepancy.

The verification information (for step 100) may come from a number ofdifference sources including, but not restricted to metrology targetcross sections such as cleaving or focused ion beam cross sections, SEM(scanning electron microscopy) and HR (high-resolution)-SEM imagescreated from such cross-sections, optical CD (Critical Dimension)metrology performed either on the metrology target itself or on anadditional proxy. These measurements may be made at various stages inthe wafer manufacturing process. For example, they may be madeimmediately subsequent to lithographic processing of a first processlayer, or subsequent to additional process steps such as etch,deposition or polish. In one embodiment the measurements are performedimmediately prior to the second lithographic step so that the previouslayer is fully processed and representative of the previous layermetrology structure, but the current layer has yet to be printed.

The stack and topography verification by OCD metrology can be done byseveral methods and may be divided between film and OCD due to differentrequirements for each measurement type. The fundamental aspect may be tomeasure the relevant stack for the OVL target due to the stack variationdifference between the device and the target which is generally placedin scribe line areas and at feature sizes and pitches different from thedevice, which affect significantly the manufacturing process impact onthe target topography. Film measurements may be carried out at an openarea/zone near the target (e.g., an OVL, overlay, target) and/orspecifically at a film pad (if such exists). The OCD or filmmeasurements may be carried out at first on special target(s) that maycontain the same pitches and geometrical design such as the (OVL) targetitself, the target being in a specific size that fits the OCD or filmtool requirements, for example several designs may be applicable, filmonly, grating over film, grating over grating and the next design may bethe OVL target itself which may eliminate the source of error due todifferent measurement location. The OCD or film targets may havedifferent spatial sizes, and may have various designed surroundingstructures. The sampling of the film or OCD metrology may be at a singlesite on the field, on a single site on the wafer, or may be full fieldand wafer level sampling. More intensive spatial sampling of locationson the wafer, or wafers in a lot may also be used to quantify theprocess variation of said topographical parameters across the wafer oracross the lot. The OCD metrology may be performed by various modelingmethods, including regression and various algorithms (e.g.,Levenberg-Marquet (L-M) and N2X solver). In certain embodiments, only asubset of the steps that are illustrated in FIG. 1 may be implemented,for example, verification 100 may enable omission of steps such ascross-section check 72, as there is already strong evidence that OCD mayreplace CD-SEM, and even achieve with better precision, and as OCD hasthe advantage of being non-destructive.

Certain embodiments of stack and topography verification sequences areshown in FIG. 2 , including additional steps which specifically relateto the use of a verification target for the purpose of stack andtopography verification. Verification target(s) may be configured tocomprise overlay target features and be size-optimized to be measureableby an OCD sensor, and be measured to derive the used OCD data.Location(s), size(s) and a proximity(ies) to surrounding structures ofthe verification target(s) may be optimized to enhance the accuracy ofthe correction provided by their measurements.

Upon receipt of target coordinates 164 from manufacturer 60, metrologyvendor 70 may prepare dedicated OCD or film verification recipes 120,measure the verification recipes 122 and use measured parameters (suchas n, k, t etc.) in an additional verification step 126 to improvefurther the updated stack description 116 achieved above. Here too, onlya subset of the steps may be implemented, for example, verification 100may enable omission of steps such as cross-section check 72, asexplained above.

Advantageously, while current simulation-based metrology target designis performed without stack and topography verification, resulting inpotentially incorrect target geometry selection, the proposed modelverification methodology enables matching between simulated and measuredmetrology performance to be achieved, resulting in more accurate targetdesigns to be achieved on subsequent tape outs. This methodology may bethe enabler for accurate metrology target design, which is a criticalrequirement for current generation metrology, specifically overlaymetrology.

It is noted that the methodology may be applied to any metrologytechnique, any metrology tools and configurations, any light source andillumination configuration, any target types and target locations, anymeasurement configuration and methodology known in the art, some ofwhich described in the background section above; and is not limited tothe hardware, algorithm/software implementations and architectures, anduse cases summarized above. Target characteristics may comprise any of:critical dimensions, overlay, sidewall angles, shape, stress,composition, films, film thicknesses, bandgap, electrical properties,process-related parameters (e.g., focus and/or dose), generating processparameters (e.g., resist state, partial pressure, temperature, focusingmodel as well as locations, sizes and surrounding structures.

FIG. 3A is a high level schematic illustration of a prior art overlaymetrology target 90. Overlay target 90 comprises four cells 91, eachdiagonal pair designed to provide overlay measurements in a differentmeasurement direction (perpendicular x and y directions). Each cell 91comprises a periodic structure 92 at a pervious (lower) layer and aperiodic structure 93 at a current (upper) layer, with periodicstructures 92, 93 being offset in opposite directions at the cells ofeach diagonal pair (e.g., the upper structure shifted at +f₀ and −f₀with respect to the lower structure). It is noted that prior art overlaytargets may comprise a different number of cells and layers, dependingon its specific design. This is shown by way of example and it isappreciated that the metrology target could also be an imaging overlay,or a so-called side-by-side grating target (as opposed to a grating overgrating metrology target illustrated in FIG. 3A).

FIGS. 3B and 3C are high level schematic illustrations of verificationtargets 130A, 130B, according to some embodiments of the invention.Verification targets 130A, 130B comprise a single cell 131 with at leastone layer having overlay target features, such as periodic structures132, 133. In the case of film measurements verification, the cell maycontain the film stack only. Cell 131 has a cell side dimension of atleast 10 μm, e.g., 10 μm, 15 μm, 30 μm, etc. and is size optimized sothat it can be measured by an OCD sensor. Verification targets 130A maycomprise at least two periodic structures 132, 133 that are identical inpitch and CD to periodic structures in one of the cells of acorresponding metrology target (such as periodic structures 92, 93 incell 91 of overlay target 90). In certain embodiments, one of offsets+f₀ or −f₀ in target 90 may be designed between periodic structures 132,133 in the corresponding verification target. For example, one of cells91 in target 90 may be enlarged to dimensions that are measureable by anOCD sensor, to form verification targets such as verification targets130A, 130B. The verification target may be designed and produced next tothe corresponding overlay target. Verification targets 130B may consista single periodic structure 132 that is identical in pitch and CD to aprevious layer periodic structure of a corresponding metrology target(such as periodic structure 132 of overlay target 90). For example,single periodic structure 132 may be a previous layer of a correspondingoverlay target, as explained above.

It is noted that in certain embodiments, the verification target(s) maycomprise corresponding verification cell(s) having only the film stack,for performing film measurements verification.

Any of the verification targets may be designed and produced next to thecorresponding metrology target and target design files may comprise adesign of any of the verification targets next to a design of thecorresponding metrology target. OCD measurements of any of theverification targets may be used in the disclosed methods and areconsidered part of the current invention.

The verification targets may comprise features, pitches andsegmentations and/or topography which are nominally identical to thoseof a single cell in the overlay target. The size and/or layout of theoverlay target may be selected to be measured on an overlay metrologysensor and the size and/or layout of the verification target may beselected to be measured on a CD metrology sensor. The verificationtargets may optionally be set next to the corresponding overlay targetsand used for e.g., lot dispositioning and/or process control feedback.

FIG. 4 is a high level flowchart illustrating a metrology target designmethod 200, according to some embodiments of the invention. Method 200may be at least partially implemented by at least one computerprocessor, e.g., in a metrology target design module. Certainembodiments comprise computer program products comprising a computerreadable storage medium having computer readable program embodiedtherewith and configured to carry out of the relevant stages of method200. Certain embodiments comprise target design files of respectivetargets designed by embodiments of method 200.

Metrology target design method 200 may comprise using OCD data relatedto at least one designed metrology target as an estimation of adiscrepancy between a target model and a corresponding actual target ona wafer (stage 210) and adjusting a metrology target design model tocompensate for the estimated discrepancy (stage 220). In certainembodiments, the used OCD data may relate to a previous layer of the atleast one designed metrology target (stage 212) and/or the used OCD datamay comprises workbook data and/or simulation results (stage 214).Method 200 may comprise estimating the discrepancy with respect togeometrical and/or optical parameters, and/or process variationparameters (stage 216). For example, any of the following aspects ofpossible sources for discrepancies may be verified by method 200: one ormore critical dimension(s), one or more optical materialcharacteristic(s) such as refractive indices (n's) and extinctioncoefficients (k's), one or more film thickness(es), topography andpresence of process variation, the latter relating to any of: inducedtopography, deposited topography, etched topography, and CMP (ChemicalMechanical Planarization) dishing; and may comprise symmetric and/orasymmetric process variation.

In certain embodiments, method 200 may further comprise designing atleast one verification target configured to comprise overlay targetfeatures, and be size optimized to be measureable by an OCD sensor,e.g., have a cell side dimension of at least 10 μm, 15 μm, 30 μm, etc.(stage 230). For example, method 200 may comprise designing theverification target(s) as enlarged cells of the overlay target(s) (stage232) and optionally designing the verification target(s) to be next tocorresponding overlay target(s) (stage 234). The at least oneverification target may be designed to be next to the at least oneoverlay metrology target. In certain embodiments, method 200 maycomprise using, for film measurements verification, a verification cellhaving only the film stack (stage 236).

Method 200 may further comprise measuring the at least one verificationtarget by an OCD sensor, e.g., by optical or SEM CD metrology, to derivethe used OCD data (stage 240). In certain embodiments, designed featuresof the verification target(s) may comprise at least one of: one or morepitch(es), one or more segmentation(s) and one or more topography(ies)which are nominally identical to corresponding feature(s) of at leastone overlay metrology target as the designed metrology target(s).

In certain embodiments, method 200 may further comprise optimizing atleast one of: a location, a size and a proximity to surroundingstructures of the at least one verification target (stage 250).

In certain embodiments, method 200 may further comprise improving amatch between measurement and simulation of overlay target(s) by usingthe measurement of verification target(s) designed using elements of theoverlay target(s) which are used to perform overlay metrology (stage260). Method 200 may further comprise improving a design of subsequentlyprinted metrology targets using the improved match (stage 262).

In certain embodiments, method 200 may further comprise optimizing ametrology recipe setup of overlay measurements of overlay target(s) byusing the measurement of verification target(s) designed using elementsof the overlay target(s) (stage 270), e.g., by feedforwardinginformation to a simulation system and/or to a respective metrologysystem or tool. For example, optimizing the metrology recipe setup maybe carried out with respect to any of: metrology accuracy, precisionand/or robustness, and/or combinations thereof (stage 272). In certainembodiments, the simulation system may be used to design metrologytargets in accordance with the method described in U.S. Pat. No.8,214,771, which is incorporated herein by reference in its entirety.

In the above description, an embodiment is an example or implementationof the invention. The various appearances of “one embodiment”, “anembodiment”, “certain embodiments” or “some embodiments” do notnecessarily all refer to the same embodiments.

Although various features of the invention may be described in thecontext of a single embodiment, the features may also be providedseparately or in any suitable combination. Conversely, although theinvention may be described herein in the context of separate embodimentsfor clarity, the invention may also be implemented in a singleembodiment.

Certain embodiments of the invention may include features from differentembodiments disclosed above, and certain embodiments may incorporateelements from other embodiments disclosed above. The disclosure ofelements of the invention in the context of a specific embodiment is notto be taken as limiting their use in the specific embodiment alone.

Furthermore, it is to be understood that the invention can be carriedout or practiced in various ways and that the invention can beimplemented in certain embodiments other than the ones outlined in thedescription above.

The invention is not limited to those diagrams or to the correspondingdescriptions. For example, flow need not move through each illustratedbox or state, or in exactly the same order as illustrated and described.

Meanings of technical and scientific terms used herein are to becommonly understood as by one of ordinary skill in the art to which theinvention belongs, unless otherwise defined.

While the invention has been described with respect to a limited numberof embodiments, these should not be construed as limitations on thescope of the invention, but rather as exemplifications of some of thepreferred embodiments. Other possible variations, modifications, andapplications are also within the scope of the invention. Accordingly,the scope of the invention should not be limited by what has thus farbeen described, but by the appended claims and their legal equivalents.

What is claimed is:
 1. A metrology target design method comprising:using optical critical dimension data related to at least one designedoverlay metrology target as an estimation of a discrepancy between atarget model and a corresponding actual target on a wafer, wherein theoptical critical dimension data is at least partially derived from ameasurement, with a critical dimension metrology sensor, of at least oneverification target comprising one or more overlay target features,wherein the one or more overlay target features of the at least oneverification target comprise at least one of: a pitch, a segmentationand a topography which is identical to a corresponding feature of atleast one overlay metrology target as the at least one designed overlaymetrology target; and adjusting a metrology target design model tocompensate for the discrepancy, wherein the adjusting is carried outwith respect to at least one of a critical dimension, an opticalmaterial characteristic, a film thickness, topography or presence ofprocess variation, wherein the process variation relates to at least oneof an induced topography, a deposited topography, an etched topography,or chemical mechanical planarization dishing.
 2. A verificationmetrology target comprising: a single cell with at least one layerhaving overlay target features, wherein the single cell has a cell sidedimension of at least 10 μm, wherein optical critical dimension data isderivable from a measurement, with a critical dimension metrologysensor, of the overlay target features of the single cell, wherein oneor more overlay target features comprise at least one of: a pitch, asegmentation and a topography which is identical to a correspondingfeature of at least one overlay metrology target as the at least onedesigned overlay metrology target.
 3. The verification metrology targetof claim 2, wherein the single cell comprises: a single periodicstructure that is identical in a pitch and a CD to a previous layerperiodic structure of a corresponding metrology target.
 4. Theverification metrology target of claim 2, wherein the single cellcomprises: at least two periodic structures that are identical in apitch and a CD to periodic structures in one of the cells of acorresponding metrology target.
 5. The verification metrology target ofclaim 2, wherein the verification metrology target is designed andproduced next to the corresponding metrology target.
 6. A metrologytarget design method comprising: using film data related to at least onedesigned overlay metrology target as an estimation of a discrepancybetween a target model and a corresponding actual target on a wafer,wherein the film data is at least partially derived from a measurement,with a critical dimension metrology sensor, of at least one verificationtarget comprising a film stack which is identical to a correspondingfeature of the target model; and adjusting a metrology target designmodel to compensate for the discrepancy.
 7. The method of claim 6,wherein the adjusting is carried out with respect to at least one of: acritical dimension, an optical material characteristic, a filmthickness, a topography, or a presence of process variation.
 8. Themethod of claim 7, wherein the process variation relates to at least oneof: an induced topography, a deposited topography, an etched topography,or chemical mechanical planarization dishing.
 9. The method of claim 8,wherein the process variation comprises symmetric and/or asymmetricprocess variation.
 10. The method of claim 6, wherein the film datarelates to a previous layer of the at least one designed overlaymetrology target.
 11. The method of claim 6, wherein the film datacomprises at least one of workbook data or simulation results.
 12. Themethod of claim 6, wherein the at least one verification targetcomprises overlay target features, wherein the method further comprises:measuring the at least one verification target with an OCD sensor toderive the film data.
 13. The method of claim 12, further comprising:optimizing at least one of: a location, a size or a proximity tosurrounding structures of the at least one verification target.
 14. Themethod of claim 13, wherein the measuring is carried out by optical orSEM CD metrology.
 15. The method of claim 13, wherein the overlay targetfeatures of the at least one verification target comprise at least oneof: a pitch, a segmentation and a topography which is identical to acorresponding feature of at least one overlay metrology target as the atleast one designed overlay metrology target.
 16. The method of claim 6,further comprising: improving a match between measurement and simulationof the at least one overlay target by using a measurement of the atleast one verification target.
 17. The method of claim 16, furthercomprising: improving a design of subsequently printed metrology targetsusing the match.
 18. The method of claim 6, further comprising:optimizing a metrology recipe setup of overlay measurements of the atleast one overlay metrology target, by using a measurement of the atleast one verification target.
 19. The method of claim 18, wherein theoptimization is carried out with respect to at least one of: a metrologyaccuracy, a metrology precision, a metrology robustness, or acombination thereof.